My forman gave me this question . Maybe he just play'm with me but here it goes:

You have 5 wires run from the basement of a building to the roof. Your on the roof. You can go to the basement and come back to the roof only once. How can you use a continuity tester to determine which wire in the basement is the same wire on the roof. In otherwords, how could you ring out all five wires by only going from the roof to the basement and back to the roof.. -Erik

It's quite possible to identify the wires with just a continuity tester and one trip.

You'll probably need to go get a pencil & paper to follow along...

Starting on the roof, link together any two of the five wires. Now select another two of the remaining three wires and link those together. Identify the remaining open wire as #1, then start your journey down to the basement.

Now, at the other end of the cable, start probing with your continuity tester until you find a pair of wires which complete a circuit. Label these wires #2 and #3 (it doesn't matter which way round; we'll sort that out later). Now find the other pair of wires which gives continuity and label those wires #4 and #5. By default, the remaining wire must be #1.

Thus far, we have #1 identified at both ends and the other four wires identified to the level of two pairs.

Still in the basement, link #1 to #2. Link #3 to #4. We now have all five wires daisy-chained up and down the building. Go get a cup of coffee to fortify yourself for the climb back to the top!

Back on the roof, remove both links you made earlier, but remember which wires were paired by the links. Now, out with the tester again and find which wire has a circuit to #1. Identify this wire as #2. The wire which was previously linked to #2 must, logically, be #3.

Only two to go now. With one probe on #3, find which of the two remaining wires has continuity and label it as #4. The last wire must be #5, of course.

This method has identified the wires and proved that all five have continuity from roof to basement.

First read them for voltage or (lick the wire, only for expert electricians or bosses). No voltage, check continuity, take your first trip downstairs and hook all wires together and check continuity then you will know. After that check for branch circuits.

Yes, I would certainly check for any existing shorts or connections between wires before starting as well. I agree that your suggestion would check the continuity of all five wires, but I took the problem to be much more than that.

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How can you use a continuity tester to determine which wire in the basement is the same wire on the roof.

I took this to mean that the problem also entailed identifying the wires at each end.

WOW, that was smart. I had to draw out a picture for that one. But sure enough, that works.

Here was my thinking:

on the roof: tie 3 of the 5 wires together. ground out one wire. leave the last wire open.

down in basement: identify ground wire as having continuity with ground. identify open wire as having no continuity. The other three wires will read continuity between them. tie the identified ground to one of the three remaining unidentified wires. and ground out another of the unidentified ground wires.

Back up on the roof: Identify which of the three wires is open, grounded, and tied to the original grounded wire.